Intramolecular Migration of Bulky Substituents in the Solid State: Vinylogous Pinacol Rearrangements Induced Thermally and by Acid Catalysis

A vinylogous pinacol rearrangement, involving an intramolecular 1,4-migration of the bulky thienothienyl substituent and a marked change of the crystal structure, was thermally induced in the solid state for thienothienyl-substituted 9,10-dihydroxy-9,10-dihydroanthracenes, 1 and 3 ∼ 8 . When heated over 180 to ∼240 °C, these crystals were transformed quantitatively, accompanying dehydration, to crystals of the anthrone derivatives without wetting or melting. Diol 8, which bears two isomeric thienothiophene rings, afforded only a single anthrone product. Compounds 1 and 8 formed a mixed crystal and its thermally induced reaction yielded two anthrone products that were attributable to intramolecular migration but afforded no anthrone derivative having the two substituents possible for the intermolecular combination. These results, along with the X-ray crystal structure of the mixed crystal, demonstrate that the rearrangement in the solid state proceeds intramolecularly. The 1,4-pinacol type rearrangements were also induced, along with the cis − trans isomerization, by cogrinding the diols with p -toluenesulfonic acid in a mortar at room temperature. The intramolecularity of the rearrangement was again proved by the use of the mixed crystal. The intervention of the carbocation in the solid-state grinding was ascertained by the UV/vis spectra. The time courses of the product distributions showed discrete profiles between the trans and cis diols, 1 and 3, respectively; for the trans diol 1, fast isomerization to the cis isomer and consecutive formation of the rearranged product via the cis isomer were observed. On the basis of these observations, the solid-state 1,4-rearrangement was deduced to occur preferentially via the cis configuration of the dihydroxy groups.